The present invention relates to a pneumatic circuit for pneumatic gauging.
As is well known, pneumatic gauging is based on the principle of either measuring the flow of compressed air to a nozzle disposed in close proximity to a workpiece surface, such as to determine the dimensional relationship between the nozzle outlet and the surface, or by measuring back pressure in a chamber through which compressed air is flowing via a restricted orifice, the back pressure being a function of the distance separating a nozzle connected to the chamber from the surface of the workpiece. In order to achieve precise measurements, the pressure of the compressed air flowing into the flow chamber in flow measuring circuits, or flowing into the pressure chamber in back pressure circuits, must be precisely regulated.
In back pressure pneumatic gauging circuits, a pressure gauge zero or datum control is provided by means of an adjustable bleed orifice from the back pressure chamber to atmosphere.
Differential back pressure circuits have been designed for the purpose of decreasing the effect of variations of pressure of the compressed air supplied to the pneumatic circuit, due to poor air pressure regulator performance. Differential back pressure pneumatic circuits consist of two air flow branches from an inlet to separate outlets. The flow of air to the pressure chamber is regulated in each branch by a fixed control orifice, and one of the pressure chambers is connected to the pneumatic gauge nozzle. The other pressure chamber is vented to atmosphere through a variable adjustable orifice providing the zero or datum control. An expansible bellows is connected to each pressure chamber, or a flexible diaphragm separates the two pressure chambers, and the differential motion of the bellows, or the deflection of the diaphragm in one direction or the other, is detected and measured as a function of the difference of pressure in the pressure chambers, which in turn represent the dimensional relationship between the outlet of the air gauge nozzle and the surface of the workpiece. As fluctuations in the supply air pressure due to poor air pressure regulator performance are present in both branches of the circuit, they tend to cancel each other such that relative accuracy of measurements is maintained in spite of slight variations in the supply air pressure.
The measured air pressure is displayed relative to a scale calibrated in dimensional units such as thousandths of an inch or millimeters. The scale is generally vertically disposed on or along a glass tube in air flow responsive circuits and in air pressure responsive circuits utilizing a U-tube manometer as a measurement display instrument. Pointer-dial dimension display instruments may be mechanically connected to the moving portion of the bellows or flexible diaphragm of differential back-pressure circuits, through pivot and lever mechanisms. Although pointer-dial instruments such as Bourdon-tube pressure gauges, diaphragm-type pressure gauges and capsule-type pressure gauge could be used directly connected to the pneumatic circuit pressure chamber, for the sake of convenience the measurement display instrument is remotely located and is either pneumatically connected to the air pressure chamber, or electrically connected to a pressure transducer disposed in communication with the pressure chamber.
In most modern pneumatic gauging systems, the measurement display instrument is electrically driven, even though it may be in the apparently traditional form of a vertical scale instrument. The pressure transducer generally takes the form of a metallic diaphragm, bellows or capsule which, directly or through a series of motion amplifying levers, displaces the input member of a probe such as a piezo resistor, a variable resistor such as a linear potentiometer, or, in the majority of systems, a displacement transducer taking the form of a variable transformer having a movable core for varying the mutual inductance between the primary and secondary windings of the transformer as a function of the displacement of the core. The input member of the displacement transducer displaces the movable core. The primary winding of the transducer transformer is connected across a power supply providing ten volts peak-to-peak, for example, at a predetermined frequency in the range generally of 5 to 10 khz. The amplitude of the signal output voltage across the secondary is a function of the linear displacement of the transformer core. The output signal, after demodulation or rectification and amplification, is applied across a voltmeter, sometimes in the form of a pointer-dial instrument or, more traditionally in pneumatic gauging technology, in the form of a vertical scale instrument imitating a U-tube manometer display or, rarely, across an analog to digital converter for providing a digital display of dimension units. The electronic circuitry providing the excitation voltage to the displacement transducer and the demodulation and amplification of the secondary signal voltage is generally installed in the housing of the remote measurement instrument, and is connected to the transducer via a multi-conductor cable provided with appropriate connectors at each end.